Ethylene alpha-olefin (polyethylene) copolymers are typically produced in a low pressure reactor, utilizing, for example, solution, slurry, or gas phase polymerization processes. Polymerization takes place in the presence of catalyst systems such as those employing, for example, a Ziegler-Natta catalyst, a chromium based catalyst, a metallocene catalyst, or combinations thereof.
A number of catalyst compositions containing single site, e.g., metallocene, catalysts have been used to prepare polyethylene copolymers, producing relatively homogeneous copolymers at good polymerization rates. In contrast to traditional Ziegler-Natta catalyst compositions, single site catalyst compositions, such as metallocene catalysts, are catalytic compounds in which each catalyst molecule contains one or only a few polymerization sites. Single site catalysts often produce polyethylene copolymers that have a narrow molecular weight distribution. Control of the molecular weight distribution and other properties can be achieved through various techniques, such as using mixed ligand metallocenes.
Monocyclopentadienyl complexes of zirconium and hafnium are precursors to mixed ligand bis(cyclopentadienyl) metal complexes (mixed ligand metallocenes), such as (n-propylcyclopentadienyl) (tetramethylcyclopentadienyl) zirconiumdichloride (hereinafter catalyst A), among others.
There are several known routes to monocyclopentadienyl complexes of zirconium and hafnium. Monocyclopentadienyl complexes of zirconium and hafnium with bulky cyclopentadienyl substituents such a tetramethyl or pentamethylcyclopentadiene are relatively easy to prepare. For example, pentamethylcyclopentadienylzirconiumtrichloride can be prepared from pentamethylcyclopentadienyllithium and ZrCl4 in ether.
Further, catalyst A can be made via the reaction of (tetramethyl) (trimethylsilyl) cyclopentadiene with ZrCl4 at high temperature (80-90° C.) in toluene. The product precipitates out of solution and after isolation is ready for further conversion to A by treatment with n-propylcyclopentadienyllithium as shown in the following reaction scheme:

As will be discussed in more detail below, the initially formed (tetramethylcyclopentadienyl)zirconiumtrichloride is stable under these conditions and does not disproportionate to an appreciable extent as do monocyclopentadienyl complexes of zirconium and hafnium with less bulky groups such as n-propylcyclopentadienyl.
An alternative synthesis of monocyclopentadienyl complexes of zirconium and hafnium has been reported by Lund and Livinghouse in Organometallics (1990), 9(9), 2426-7. In these methods either bis(dimethylsulfide)ZrCl4 or bis(dimethylsulfide)HfCl4 are reacted with trimethylsilylcyclopentediene and tri-n-butyltincyclopentadiene respectively at low temperature to form the monocyclopentadienyl complexes as shown in the following reaction scheme:

Monocyclopentadienyl complexes of zirconium and hafnium can also be formed via the reaction of metallocenes with the respective metal tetrahalide. This is especially convenient when the starting bis complex is commercially available or easily synthesized.
US2007/0060722 discloses a method for making (n-BuCp)ZrCl3 from bis(n-butylcyclopentadienyl)zirconiumdichloride and zirconiumtetrachloride in which the two reactants are refuxed for 20 hours in toluene, the reaction mixture is centrifuged to remove ZrCl4, and the resulting product is purified by multiple precipitations from CH2Cl2/pentane to yield the product in about 87% yield. The purity of the material is reported to have a mono to bis ratio of 52:1 as compared with the same reaction carried out at room temperature for 20 h which gave a mono to bis ratio of 1.4 to 1. There is also an example in which (1,3-butyl-methylcyclopentadienyl)zirconium trichloride is prepared in the same manner from bis(1,3-butyl-methylcyclopentadienyl)zirconiumdichloride (E dichloride) and ZrCl4 in 76% yield.
The procedures detailed above may provide a monocyclopentadienyl complex, but take significant time for completion. Further, in many of these techniques only bulky substituents on the cyclopentadiene ring slow or prevent disproportionation to a dicyclopentadienyl complex.